Optical instrument for spying purposes



-June 24 1924.

, l c. HERTEL.

OPTICAL. INSTRUMENT FoavsPYING TuaPosns Filedvoct. 2o, 1922 INVENTOR (01.! f/M BY m.

ATTORNEY Pateiited June 24,i 1924.

PATEISVI OFFICE.-

CARL HERTEL, OF H ERNSDORF, NEAR BERLIN, GERMANY, ASSIGNOR T70 CHARLES FREDERICK SGHMID, F NEW ROCHELLE, NEW YORK.

OPTICAL INSTRUMENT FOR SYYING PURPOSES.

Application led October 20, 1922. Serial No. 595,849.

To all whom t may concern.' Be it known that I, CARL Hamm., a citizen of the German Republic, residing 'at' Hermsdorf, near Berlin, Germany, have invented certain new and useful Improvements in Optical Instruments for Spying Purposes, (for which I have filed patent application in Germany October 21st, 1921),

l of which the following is a specification.

It is usual to provide in doors for closin apartments, openings allowing the house holder to see persons outside the doorl without such person seeing the one inside before opening the door.

Such spying openings as hitherto used offer the inconvenience that the person inside the door or wall has not a sutliciently lar e field of view. v

'lghis invention consists in an optical device yielding an enlarged field of view, so

that a sullicient portion of a person outside the door can be seen in spite of great narrowness of the aperture: To this end the new device consists of a lens combination comprising a negative lens element of lar e diameter and a positive lens elemento small diameter. Preferably the negative lens element of large diameter is given a comparatively small focal length, whereas the small positive lens element is given a comparatively large focal length of about double theffocal length of the negative lens element.

The optical data for an embodiment of a a5 device in accordance with my invention are as follows:

Negative lens element formed as simple plano concave lens: Diameter 30 mm. Focal length mm. Refraction index 4o about 1.5.

Positive lens element consisting of a simple lano convex lens: Diameter 9 mm. Focal ength 63 mm. Refraction index about 1.5.

Distance of the lenses` from each other about 35 mm.

The new device mounted within a spying opening of a door is illustrated in the crosssectional view shown on the annexed drawing. i

The door is designated. with the reference letter a, the negative lens element is designated b, the tubular casing for the lens elements c and the positive lens element d.

The path of the rays indicated on the drawing represents the field of view which,

can be overlooked by the observer.

The instrument is to be so inserted into the spying opening of a door, wall or the like that the positive lens d is on the inside of the door and forms the eye lens of the A -said lenses lying in the same horizontal plane.

3; Optical instrument for lspying purposes comprising ra negative lens element of large diameter and a positive element of small diameter, the positive element having substantially double the focal length of the negative element, the axes of the said lenses lying in the same horizontal plane.

4. Optical instrument for spying purposes comprising a negative lens element of large diameter and a focal length of substantially 30 mm. and a positive lens element of small diameter and a focal length of substantially 60 mm., the axes of the said lenses lying in the same horizontal plane.

5. Optical instrument for spying purposes lcomprising a tubular casing a negative lens element of large diameter and a positive element of small diameter, the two elements so mounted within the casing as,

to form an image reducing system.

6. The combination with a door, wall or the like provided with an opening extending therethrough of a lens combination within said opening comprisin a negative element' of large diameter an .a positive element of small diameter, ,the two elements so arranged with relation to each other as4 to form an image producing s stem.

7. The combination with a oor, wall or the like provided with an opening extend- In testimony whereof I have signed this ing therethrough of a lens combination specification in the presence of two subscribwithin said opening comprising a negative ing witnesses. element of large diameter and small focal 5 length /and a. ositive element of small Witnesses:

diameter and su tantially double'the focal E. H. HERMER, length of the negative element. G. Kunscmswrrz.

CARL' HERTEL. 

